A method of this type is known from DE-PS 28 31 702. In this known method, preheating is effected by an additional energy source which comprises a plasma arc of a non-fusible electrode. At least part of the preheating plasma arc of the non-fusible electrode passes obliquely into an arc of a fusible electrode.
A disadvantage of this method is that it cannot be applied to age-hardened superalloys, since the area of effect of the additional energy source, i.e. the plasma arc, is extremely limited and its temperature cannot be controlled accurately.
A method of welding, melting or heating a workpiece by means of light energy is known from DE 22 57 739. This method is used in a modified manner for preheating superalloys using a protective gas. In this modified method the light energy is not concentrated on the object to be heated as in DE 22 57 39, but parallel radiators heat the workpiece over a large area. In this method, welding is not carried out by means of light energy, but by arc welding using a protective gas.
A disadvantage of this method is that parallel radiators are limited with respect to size and heating capacity so that the entire workpiece is heated to the preheating temperature and considerable preheating times of more than 5 minutes are necessary. At the same time, the housing containing the workpiece, the welding electrode and the protective gas is, disadvantageously, also partly heated. Finally, the light source represents a high cost factor because of its high power density, as the life of the light radiators is severely restricted by the high output required. Furthermore, in this method not only is the welding region of the workpiece heated, but the protective gas is also heated by convection via the irradiated protective gas housing with the result that there is considerable generation of heat in the work environment during the preheating and welding operations. This method involves high energy consumption. Furthermore, cooling means are necessary in particularly critical areas of the protective gas housing or container.
A method of repairing turbine blades by means of the deposition welding of superalloys is disclosed in U.S. Pat. No. 4,141,127. In this method, the welding stresses are subsequently reduced by a tempering process at 1200.degree. C. for 1 hour. A disadvantage of this method is that this repair process with its subsequent tempering cannot be used for all precipitation-hardened superalloys. Furthermore, there is the danger of crack formation when passing through the temperature range of what is known as the ductility hole with .gamma.' constituents, and of workpiece distortion during welding and tempering.